1. Field of the Invention
The present invention relates to an image heating apparatus and a fixing apparatus which are used in a copying machine, a printer, a facsimile, and the like each adopting an electrophotographic system or an electrostatic recording system.
2. Related Background Art
Heretofore, one of the factors affecting an impression of a user with respect to an image outputted by an electrographic apparatus is glossiness. The glossiness preferred by a user differs depending on the user and the type of output image. In general, a text image such as an image formed of sentences is preferred to have a lower glossiness, while a graphic image such as a photograph is preferred to have a higher glossiness. However, the conventional glossiness of the image after completion of the fixing process inherently differs depending on the apparatus, and thus a user cannot select the desired value of glossiness by a high precision manner.
Here, the glossiness depends on a surface characteristic of a toner image after completion of the fixing process. In the conventional image forming apparatus, the surface characteristic depends on a material of a toner, a construction of a fixing apparatus, and the like. More specifically, the surface characteristic of a toner image after completion of the fixing process is determined during the fixing and separation. That is, the toner which is sufficiently dissolved during the fixing process is then separated so as to have the surface characteristic equal to that of a fixing roller, and thus its glossiness becomes high. On the other hand, when the toner is separated in a state where the toner is not perfectly dissolved during the fixing process, its surface becomes rough and its glossiness becomes low. However, when the toner is sufficiently dissolved during the fixing process, a high temperature offset is apt to generate, while when the toner is not perfectly dissolved, imperfect fixing is apt to generate.
As regards the factors determining the fixing characteristic, there are given a fixing temperature, a fixing nip width (a nip length along a recording material conveyance direction), a fixing speed, and the like. Thus, when the fixing temperature is set so as not to generate the imperfect fixing and the high temperature offset, the glossiness is determined accordingly. Actually, such a latitude of the fixing temperature as not to generate the imperfect fixing and the high temperature offset is narrow, and has about 10° C. for example. Hence, if the latitude is changed with this range, the glossiness hardly changes.
In addition, conventionally, an image forming apparatus is known which can select different levels of the glossiness by changing a fixing speed. However, in this case, the number of selectable levels of the glossiness depends only on the number of changeable fixing speeds. Thus, when there are two kinds of fixing speeds, two kinds of selectable levels of the glossiness are obtained accordingly. Though there are two kinds of fixing speeds, a user merely selects between the two kinds of levels of the glossiness which are determined based on the apparatus construction in advance. In addition, when the fixing speed is made slow, there arises a problem in that it takes time to output a toner image.
In order to cope with such problems, an image forming apparatus disclosed in Japanese Patent Laid-Open No. 2002-365967, as shown in FIG. 14, adopts a construction in which a toner image on a recording material is fixed by a first fixing unit A′ (including a first fixing roller 101 and a first pressure roller 102), and a second fixing unit B′ (including a second fixing roller 103 and a second pressure roller 104) which is provided on a downstream side with respect to the first fixing unit A′.
More specifically, a toner image t on a recording material P is fixed by the first fixing unit A′, and the glossiness of the resultant toner image is adjusted by the second fixing unit B′ provided on the downstream side with respect to the first fixing unit A′.
The adjustment of the glossiness is carried out by changing an attemperation temperature (target temperature) of the second fixing unit B′. That is, firstly, a function of the first fixing unit A′ mainly aims at “fixing”, and the setting is made in the function of the first fixing unit A′ so as not to generate the above-mentioned high temperature offset and imperfect fixing. Thus, the recording material P passes through the first fixing unit A′, whereby the toner image t has already been fixed. The surface characteristic and glossiness of the toner image t at this time have certain values, respectively.
A function of the second fixing unit B′ mainly aims at “glossiness adjustment”. Thus, the attemperation temperature of the second fixing unit B′ is set so that the glossiness of the toner image has a desired value. That is, when the attemperation temperature of the second fixing unit B′ is set to be low, the glossiness does not become very high, while when the attemperation temperature of the second fixing unit B′ is set to be high, the glossiness becomes high. As a result, the glossiness can be changed without changing the fixing speed.
However, in the case of the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2002-365967, the attemperation control for the first fixing roller 101 of the first fixing unit A′ and the attemperation control for the second fixing roller 103 of the second fixing unit B′ are carried out independently of each other.
Then, though a temperature of an outer surface of the first fixing roller 101 of the first fixing unit A′ reaches a desired temperature at the beginning of an image forming job for continuously forming an image on a plurality of recording materials, an inner side of the first fixing roller 101 of the first fixing unit A′ has not heat accumulation so much. As a result, a phenomenon occurs in which the temperature of the first fixing roller 101 decreases as the recording materials successively pass through the first fixing unit A′.
On the other hand, since the second fixing roller 103 of the fixing unit B′ is brought into contact with the recording material which has already been heated in the first fixing unit A′, a decrease in temperature of the second fixing roller 103 accompanying the progress of the image forming job is small.
As a result, the glossiness of the image formed on the recording material changes in the middle of the image forming job, and thus the appearance quality of the image is reduced.